Saddle position/setback on a new frame

[avalonracing]

You have to base all measurements from the center of the BB as this is fixed. A bike with a higher or lower BB will make everything else feel a bit different unless you measure from there. This is the kind of craziness I've gone to get positions matched. It is a negative image of my Merlin layered over my Klein when trying to match the fit. I saw once I lifted the Merlin's lower BB to match the high BB on the old Klein (with vintage criterium geometry) it was making the bar height on the Merlin higher even though the bike height was the same when measured side by side. You can see the difference between the 74º STA on the Klein vs. the 73ºSTA on the Merlin (the Merlin saddle was slid forward to match the Klein)

[OtayBW]

Originally Posted by Dave

Wavy concrete floors make for unreliable measurements with a drywall square. With a plumb bob, the floor doesn't have to be level or free of waviness, if the two bikes being compared are placed in the same spot for comparison.

Who says we're doing it on a wavy concrete floor? Geez...

[Mzilliox]

the answer is yes

your saddle will be in the same spot relative to the BB, which is where you want to be for pedaling efficiency.

now does this mean you may feel further forward or backward in relation to the front center? or front wheel? sure, the rear wheel may be further back or further forward on one bike. the fork on one bike may have more or less rake, putting the front wheel further from you, or closer to you, all of these things could make you feel like you are sitting further back on the madone.

so maybe the madone has a rear wheel tucked in closer? or maybe the head tube is more slack?

im kinda surprised theres that much difference

[Dave]

Originally Posted by OtayBW

Who says we're doing it on a wavy concrete floor? Geez...

I've never seen a concrete floor that wasn't wavy. They screed the concrete with a 2×4, then hand trowel the concrete to make a smooth surface, but it's never very flat.

Get a 6 foot level and place it on the floor. With feeler gauges, you'll probably find some significant ups and downs.

Working in the machine shop business for many yeas gives me an appreciation for a flat surface.

Building a house showed me how uneven most concrete floors really are.

If you laid a 4x8 of 3/4" MDF on the floor, it would make a more suitable measuring surface.

[benb]

Originally Posted by Dave

Wavy concrete floors make for unreliable measurements with a drywall square. With a plumb bob, the floor doesn't have to be level or free of waviness, if the two bikes being compared are placed in the same spot for comparison.

The plumb bob is worse if the floor is not level if you start doing the math for what kind of error is induced if the floor is not level.

The plumb bob is hanging from a long piece of string from the saddle down past the BB. The saddle setback is a small # compared to the length of string, so if the floor is not level the error is a high % of the actual setback.

If you measure out from the wall you might not get the measuring tape/stick level but one side of the triangle (height) is very very small if you're not holding the measuring device level.. you're making a very small change in the hypotenuse of the triangle and that's the error. The error is very small compared to the length from the BB/saddle tip to the wall.

For the height.. I was never advocating measuring that up from the floor. Do the math for your STA and measure up from the BB through the saddle. But realize the distance needs to change on different bikes with different STAs so that you keep the Y distance the same.

I see your point though that if you put the 2 bikes in the same spot on the same floor that reduces the error on the plumb bob. But what if the bikes have different wheelbases and are differently affected by the wavy floor? You're back to not controlling that.

Obviously a lot of this is overthinking things! But plumb bobs also get caught on chainstays and such and don't hang quite vertical too.

I think I only have a 3' level at max and that's enough to easily see the floor is all kinds of wavy and not level in the garage.

[OtayBW]

Originally Posted by Dave

I've never seen a concrete floor that wasn't wavy. They screed the concrete with a 2×4, then hand trowel the concrete to make a smooth surface, but it's never very flat.

Get a 6 foot level and place it on the floor. With feeler gauges, you'll probably find some significant ups and downs.

Working in the machine shop business for many yeas gives me an appreciation for a flat surface.

Building a house showed me how uneven most concrete floors really are.

If you laid a 4x8 of 3/4" MDF on the floor, it would make a more suitable measuring surface.

Who says I'm even on concrete? What makes you think that I have not considered all of of this already? I already use a 6 ft level and I work in a suitable place, and I am aware of the precision and bias in the measurements. I also don't care for the plumb bob method, but that's just me. Geez man....

[Dave]

Originally Posted by OtayBW

Who says I'm even on concrete? What makes you think that I have not considered all of of this already? I already use a 6 ft level and I work in a suitable place, and I am aware of the precision and bias in the measurements. I also don't care for the plumb bob method, but that's just me. Geez man....

Excuse me! I must have read that into your post. In that case, it's something for those working in the basement or garage to consider. I've never worked on a bike on any other type of floor.

The spot in my workshop where I make this sort of measurement shows perfectly level over 6 feet, but it's sure got some up and downs over that length.